Optical coherence tomography (OCT) is a technique for obtaining high resolution information about the internal structure of a transparent object, such as the retina of the eye. The object is scanned with a laser beam from an interferometer. The scanning beam is typically generated by an interferometer with a broadband light source so that the coherence length of the light is relatively short, typically in the order of 2 microns. In time domain OCT, image information is generated from the region, known as the coherence gate, where the optical path difference between the reference beam and object beam is such that it is length than the coherence length of the light. By changing the optical path difference to move the coherence gate in the depth direction, it is possible to obtain image information from the object in this direction. A simple line scan in the z direction is known as an A scan, a two-dimensional or cross-sectional scan in the z-direction, so as to obtain a vertical or horizontal slice extending in the depth direction, is known as a B scan, and an en-face scan across the object is known as a C-scan.
While optical coherence tomography produces very high resolution images in the depth direction, it is difficult to relate the OCT imaging position with an overall view of the eye. For this purpose it is known to superimpose an SLO (scanning laser ophthalmoscope) image on the OCT image. An SLO en-face image is generated by a confocal scanner and while giving poorer resolution than the OCT image gives a more recognizable image of the retina of eye. The SLO image can be used to guide the OCT examination and permit the user to register precisely where the OCT image was taken on the eye fundus.
In conventional time-domain OCT imaging, it takes in the order of ½ second to obtain an OCT frame for a typical B-scan. The SLO image can be generated in about ½ second. The delay in creating the OCT image leads to inaccuracies in the registration of the OCT image against the SLO image because of potential movement of the eye fundus between the creation of the images. One possible solution to this problem is to use a full-field flash image instead of an SLO image. While this process captures the whole image area at once, it cannot be used continuously and lacks the versatility of SLO imaging for registration purposes.
Typically, such combined systems that employ both a confocal and OCT scanner involve complex optics. Alternatively, it is possible to derive a pseudo confocal image in software from the OCT signal, but such an image is not as good as a true confocal image since it depends on the OCT signal, and multiply backscattered light does not contribute to the OCT signal.
Fluorescence imaging is a technique that is commonly used in the imaging of biological samples. For example, it can be used to study biological processes occurring within the retina. In fluorescence imaging, the sample is illuminated with a light of one wavelength, which causes fluorophores, such as ICG (indocyanine green) in the sample to fluoresce at a different wavelength from the illuminating light. The fluorescent light is detected and used to form an image, which gives information about internal biological processes occurring within the sample.
Fluorescence imaging can be combined with an OCT image to look at biological processes within the eye in the context of a three dimensional scan of the eye. While it would be desirable to produce a scan covering a three dimensional volume of the sample at the same time as a fluorescent image is produced, this is not possible with conventional time domain OCT imaging because of the time it takes to create an OCT three dimensional image. The fluorescence image is created as a raster scan of the surface of the object in the x-y plane. It is not possible to simultaneously obtain depth imaging for the whole image and a fluorescence image using conventional time domain OCT technology due to time constraints.